Magnetic field sensors for detecting movement of a ferromagnetic object are known. The magnetic field associated with the object is detected by one or more magnetic field-to-voltage transducers (referred to herein as a magnetic field sensing elements), such as Hall effect elements or magnetoresistance elements, which provide one or more signals (i.e., magnetic field signals) dependent on a magnetic field associated with the object.
Some magnetic field sensors merely provide an output signal representative of the proximity of the object and maybe referred to as proximity detectors. However, other sensors, e.g., rotation detectors, provide an output signal representative of the approach and retreat of features of a rotating object, such as each tooth of a rotating gear or each segment of a ring magnet having segments with alternating polarities. The rotation detector processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal either reaches a peak (positive or negative peak) or crosses a threshold level. Therefore, the output signal, which has an edge rate or period, is indicative of a speed of rotation of the gear or of the ring magnet.
The magnetic field signal is dependent on the distance between the object, for example the rotating ferromagnetic gear, and the magnetic field sensing element(s), for example, the Hall elements. This distance is referred to herein as an “air gap.” As the air gap increases, the magnetic field sensing element tends to experience a smaller magnetic field from the rotating ferromagnetic gear, and therefore smaller changes in the magnetic field generated by passing teeth of the rotating ferromagnetic gear.
In one type of rotation detector, sometimes referred to as a peak-to-peak percentage detector (or threshold detector), a threshold level is equal to a percentage of the peak-to-peak magnetic field signal. For this type of sensor, the output signal changes state when the magnetic field signal crosses the threshold level. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold,” which is assigned to the assignee of the present invention and incorporated herein by reference in its entirety.
In another type of rotation detector, sometimes referred to as a slope-activated detector or as a peak-referenced detector, threshold levels differ from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of sensor, the output signal changes state when the magnetic field signal departs from a peak and/or valley by the predetermined amount. One such slope-activated detector is described in U.S. Pat. No. 6,091,239 entitled “Detection of Passing Magnetic Articles with a Peak Referenced Threshold Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference in its entirety.
It should be understood that, because the above-described peak-to-peak percentage detector and the above-described peak-referenced detector both have circuitry that can identify the positive and negative peaks of a magnetic field signal, the peak-to-peak percentage detector and the peak-referenced detector both include a peak detector circuit adapted to detect a positive peak and a negative peak of the magnetic field signal. Each, however, uses the detected peaks in different ways.
In order to accurately detect the positive and negative peaks of a magnetic field signal, some magnetic field sensors, are capable of tracking at least part of the magnetic field signal. To this end, typically, one or more digital-to-analog converters (DACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
Some rotation detectors detect a direction of rotation of the object. In one example, two or more magnetic field sensing elements are positioned in proximity to the object, adjacent to each other. The phases of the resulting output signals are separated by a phase difference associated with the spacing between the elements and the phase relationship of the output signals (i.e., the sequence of rising and falling edges) can be used to determine the direction of rotation.
Some rotation detectors are configured to identify a vibration of the rotating object, which vibration can generate signals from a magnetic field sensing element that might appear similar to signals that would be generated during a rotation of the gear or ring magnet in normal operation. Sensors having vibration processors that can detect a vibration are described in U.S. Pat. No. 7,365,530, entitled “Methods and Apparatus for Vibration Detection,” and in U.S. Pat. No. 7,253,614, entitled “Proximity Detector Having a Sequential Flow State Machine,” both of which are assigned to the assignee of the present invention and incorporated herein by reference in their entireties.
For a ferromagnetic gear capable of rotation about an axis of rotation in normal operation, the vibration can have at least two vibration components. A first vibration component corresponds to a “rotational vibration,” for which the ferromagnetic gear vibrates back and forth about its axis of rotation. A second vibration component corresponds to “translational vibration,” for which the above-described air gap dimension vibrates. Rotational vibration and the translational vibration can occur even when the ferromagnetic gear is not otherwise rotating in normal operation. Both the first and the second vibration components, separately or in combination, can generate an output signal from the rotation detector that indicates rotation of the ferromagnetic gear even when the ferromagnetic gear is not rotating in normal operation.